Interest in single-site (metallocene and non-metallocene) catalysts continues to grow rapidly in the polyolefin industry. These catalysts are more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of α-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Traditional metallocenes commonly include one or more cyclopentadienyl groups, but many other ligands have been used. Putting substituents on the cyclopentadienyl ring, for example, changes the geometry and electronic character of the active site. Thus, a catalyst structure can be fine-tuned to give polymers with desirable properties. Other known single-site catalysts replace cyclopentadienyl groups with one or more heteroatomic ring ligands such as boraaryl (see, e.g., U.S. Pat. No. 5,554,775), pyrrolyl, indolyl, (U.S. Pat. No. 5,539,124), or azaborolinyl groups (U.S. Pat. No. 5,902,866). Amine oxides are widely used in the polymer industry as stabilizers (see, for example, U.S. Pat. No. 5,268,114), and many are commercially available. Seldom, however, have amine oxides been used in a process for polymerizing olefins or as a component of an olefin polymerization catalyst. An exception is U.S. Pat. No. 4,015,060, which teaches to use sterically hindered heterocyclic amine oxides (such as pyridine N-oxide or 2,6-lutidine N-oxide) in combination with a Ziegler-Natta catalyst (titanium trichloride, a trialkyl aluminum, and a dialkyl aluminum halide) to polymerize propylene. The amine oxide reduces the amount of low-molecular-weight, alkane-soluble impurities in the desired product, crystalline polypropylene.
In contrast, single-site olefin polymerization catalysts that contain N-oxide ligands are not known. Also unknown are catalysts that incorporate a chelating N-oxide ligand, i.e., one that can form a chelate using the N-oxide oxygen atom and a second atom that can donate an electron pair to the transition metal.
The commercial availability of many N-oxides and the ease with which a host of other interesting N-oxide ligands can be prepared (e.g., by simply oxidizing the corresponding tertiary amine with hydrogen peroxide or a peracid) suggests that single-site catalysts with advantages such as higher activity and better control over polyolefin properties are within reach. Ideally, these catalysts would avoid the all-too-common, multi-step syntheses from expensive, hard-to-handle starting materials and reagents.